The first few weeks of the season saw United score five at Wigan, four against City and three at Tottenham.
But, since then, they have failed to score more than two in a league match - allowing Chelsea to build a considerable advantage in their goal difference column as well.
While United put that right on Saturday against Everton, Fergie admitted it has been weighing on his mind over the past couple of months.
He said: "I am frustrated and no doubt our supporters are too. But we are working on the problem.
"Whether the players feel that they make so many opportunities in the game that they can afford to wait, I'm not sure. It's like missing a bus but, not to worry, because there'll be another along in a minute.
"Somewhere along the line we have lost the urgency to stamp our early authority on a game in terms of scoring."